An artificial life approach to studying niche differentiation in soundscape ecology David Kadish and Sebastian Risi and Laura Beloff IT University of Copenhagen, Copenhagen, Denmark [email protected] Downloaded from http://direct.mit.edu/isal/proceedings-pdf/isal2019/31/52/1903579/isal_a_00140.pdf by guest on 28 September 2021 Abstract the audio spectrum in response to different evolutionary pressures. Through the experiment, evolutionary pressure to Artificial life simulations are an important tool in the study communicate within a species is found to play a significant of ecological phenomena that can be difficult to examine role in the formation of acoustic niches. directly in natural environments. Recent work has established the soundscape as an ecologically important resource and In examining the emergence of communication between it has been proposed that the differentiation of animal artificially evolved species, this study draws from a vocalizations within a soundscape is driven by the imperative body (Arita and Koyama, 1998; Wagner, 2000; Sasahara and of intraspecies communication. The experiments in this Ikegami, 2007) of ALife-based studies of communication paper test that hypothesis in a simulated soundscape in order to verify the feasibility of intraspecies communication including the work of Floreano et al.(2007) in emergent as a driver of acoustic niche differentiation. The impact communication between robotic agents. However, it is of intraspecies communication is found to be a significant distinct from these previous studies in its focus on the factor in the division of a soundscape’s frequency spectrum effect of the emergent communication on the ecological when compared to simulations where the need to identify phenomena of niche differentiation. signals from conspecifics does not drive the evolution of signalling. The method of simulating the effects of The main contributions of this study are the development interspecies interactions on the soundscape is positioned as of a simplified model of a soundscape for the purpose of a tool for developing artificial life agents that can inhabit and rapid experimentation and in-depth analysis of population- interact with physical ecosystems and soundscapes. soundscape dynamics, and the demonstration of the ANH on this model. Introduction Background Artificial life experiments have become important tools for exploring biological phenomena. In particular, they In the physical world, the concept of soundscape — have allowed researchers to study the relationships between the collection of the acoustic features of a landscape — evolutionary processes and ecological theories (Aguilar has roots and influences in a diverse array of academic et al., 2014), like the emergence of interspecies relationships fields (Lyonblum, 2017). It grew initially out of the arts like mutualism and parasitism (Watson et al., 2000). and cultural studies work of Westerkamp(1974), Schafer One area of ecology that has received little attention thus (1977), and Truax(1978), but has since expanded into far from artificial life (ALife) studies is soundscape ecology. the sciences. In the field of ecology, the soundscape The field of soundscape ecology has been formalized by is considered an important ecological resource and its researchers over the past decade (Pijanowski et al., 2011a), composition is thought to indicate the diversity and stability building on earlier conceptions of the soundscape (Schafer, of the ecosystem (Pijanowski et al., 2011b). Though the 1977). One of its foundational theories is the acoustic field of soundscape ecology was only proposed relatively niche hypothesis (ANH)(Krause, 1987), which applies the recently (Pijanowski et al., 2011b), the application of concept of ecological niches — the distribution of resources ecological principles to the study of soundscape has a longer that are used by a species in an ecosystem (Pocheville, 2015) history. Notably, the concept of ecological niches was — to the soundscape. first introduced in the context of sonic resources by Krause This experiment tests the proposed mechanisms for the (1987) as the acoustic niche hypothesis. formation of these niches in a virtual soundscape in order to understand how species change vocalizations in response Acoustic niche hypothesis(ANH) to one another. It models the behaviour of two species in a The acoustic niche hypothesis expands the concept of virtual ecosystem and tracks how their calls shift through ecological niches to the spectro-temporal plane of the 52 soundscape. Krause proposed that, in the same way that metrics, this study examines the mechanisms that breed niche differentiation leads to species making use of the range interspecific diversity and intraspecific convergence in the of physical resources available in an ecosystem, species tend vocalizations of communities in a soundscape. In particular, to differentiate their use of an ecosystem’s sonic resources. it is designed to test Endler’s hypothesis (1992) that the This differentiation, according to Krause, occurs spectrally ability to identify vocalizations from members of the same in the sonic frequencies that animals use for vocalization species drives acoustic niche differentiation. and temporally in the time-based patterns of their sounds. The acoustic niche hypothesis posits that soundscapes The theory holds that older, more mature ecosystems should niches are differentiated on both spectral and temporal show a greater degree of differentiation between the auditory levels, so that species ensure that their calls are isolated in niches that long-established species occupy. both frequency and time. In order to simplify the modelling The ANH describes the result of acoustic differentiation, and analysis and to allow for a deeper examination of the but Endler(1992) proposed the primary mechanism for this effects of differentiation, this study focuses only on the evolutionary driver: sexual selection based on a mate’s spectral component of this differentiation. ability to hear a call and the ability to maintain territory. Downloaded from http://direct.mit.edu/isal/proceedings-pdf/isal2019/31/52/1903579/isal_a_00140.pdf by guest on 28 September 2021 In this formulation, vocalizations and auditory receptors Approach have co-evolved to maximize the reception of signals The experimental setup for testing the drivers of acoustic from members of ones own species (conspecifics), while niche differentiation consists of a set of evolving populations minimizing interference from members of other species and a soundscape that they communicate within. The (heterospecifics). experiment tests two hypotheses: the alternative hypothesis This type of spectral differentiation has been observed (H1), that acoustic niche spectral differentiation is driven a numerous times in the wild: in the calls of certain need to identify signals from potential mates or territorial species of frogs (Feng and Schul, 2007); in the buzzing rivals of the same species; and the null hypothesis (H0) that of cicadas (Sueur, 2002); and in the overall division of a spectral differentiation in acoustic niches is not driven by the soundscape in Borneo between a series of birds, gibbons, need to distinguish the species of the signaller. and accompanying insects (Krause, 2008). However, it has In order to facilitate rapid experimentation and ease the proved difficult to experimentally probe the formation and analysis of the emergent signalling systems, the experiments division of spectral niches, due to the lengthy timescales use a simplified, discretized model of a soundscape instead that would be required to allow evolutionary processes of a full-spectrum, temporally-varying acoustic space. to progress (Miller, 1995) and the complexity of the Sounds are modelled as 9-bit vectors that represent the systems and soundscapes that are encountered “in the use of 9 available frequency bands in an instantaneous wild” (Wheeler et al., 2002). signal. These simplifications allow the repetition of the experiments many times with a large number of generations ALife approaches and individuals, such that results reflect general trends in Where ecological phenomena have been difficult to the dynamics of these systems rather than the peculiarities experimentally investigate, researchers have proposed that of any single simulation. The entire system is illustrated ALife approaches can be a mode of inquiry that allows in Figure 1 and described in detail in the sections below. for the manipulation of particular conditions and the rapid Lettering in brackets refers diagram labels in Figure 1. collection of large quantities of data about a simulated ecological system (Miller, 1995). In 2018, Eldridge and Populations Kiefer proposed synthetic acoustic ecology (SAC) as a In soundscape ecology in the physical world, the toolset for exploring questions in the field of soundscape communicative process is often assessed in two parts: ecology using ALife methods in virtual ecosystems. Their sender and receiver. Every individual, of course, is both study examined one of the assertions of ANH(Krause, sender and receiver, but the processes experience different 1987) — that one can identify the maturity of an ecosystem evolutionary pressures; “[n]atural selection favors signals by examining its acoustic signature. Using a multi-agent that elicit a response in the receiver that increases or system model, they demonstrated that patterns emerge in maintains the fitness of the sender” (Endler, 1992). The two common acoustic indices that indicate